Magnetic resonance imaging ("MRI") is a well known, highly useful technique for diagnosing abnormalities in biological tissue. MRI can detect abnormalities which are difficult or impossible to detect by other techniques, without the use of x-rays or invasive procedures.
Magnetic resonance imaging uses changes in the angular momentum or spin of the atomic nuclei of certain elements within body tissue in a static magnetic field after excitation by radio frequency energy, to derive images containing useful information concerning the condition of the tissue. During a magnetic resonance imaging procedure, the patient is inserted into an imaging volume containing a static magnetic field. The vector of the angular momentum or spin of nuclei containing an odd number of protons or neutrons tends to align with the direction of the magnetic field. Irradiating the tissue within the imaging volume by a pulse or pulses of radio frequency energy having a particular bandwidth of frequency, referred to as the resonant or Larmor frequency, shifts the vectors of those nuclei out of alignment with the applied magnetic field. The spins of the nuclei then turn or "precess" around the direction of the applied primary magnetic field. As their spins precess, the nuclei emit small radio frequency signals, referred to as magnetic resonance ("MR") signals, at the resonant or Larmor frequency, which are detected by a radio frequency antenna tuned to that frequency. Gradient magnetic fields are provided to spatially encode the MR signals emitted by the nuclei. After the cessation of the application of radio frequency waves, the precessing spins gradually drift out of phase with one another, back into alignment with the direction of the applied magnetic field. This causes the MR signals emitted by the nuclei to decay. The MR signals are detected by a radio frequency receiving antenna positioned within the imaging volume proximate the patient, and are amplified, digitized and processed by the MRI system. Hydrogen, nitrogen-14, phosphorous 31, carbon 13 and sodium 23 are typical nuclei detected by MRI. Hydrogen is most commonly detected because it is the most abundant nuclei in the human body and emits the strongest MR signal.
The rate of decay of the MR signals varies for different types of tissue, including injured or diseased tissue, such as cancerous tissue. By known mathematical techniques involving correlation of the gradient magnetic field and the particular frequency of the radio frequency waves applied at various times with the rate of decay of the MR signals emitted by the patient, it is possible to determine the concentrations and the condition of the environment of the nuclei of interest at various locations within the patient's body. This information is typically displayed as an image with varying intensities, which are a function of the concentration and environment of the nuclei of interest. Typical MRI systems are the Quad 7000 and Quad 12000 available from FONAR Corporation, Melville, N.Y., for example.
The quality of the magnetic resonance image is directly related to the characteristics of the receiving antenna. Significant electrical characteristics of the antenna include sensitivity, Q factor and the signal-to-noise ratio.
Sensitivity is the signal voltage generated in the receiving antenna by MR signals of a particular magnitude. The higher the sensitivity within the region to be imaged, the weaker the signals which can be detected. The sensitivity of the antenna is preferably substantially uniform with respect to MR signals emanating from all volume elements within the region of the subject which is to be imaged.
The Q or quality factor, which is closely related to sensitivity of the antenna, is a measure of the ability of the antenna to amplify the received signal. The Q-value of the antenna can be lowered by a patient proximate or within an antenna, due to inductive coupling between the patient and the antenna. Antennas must therefore have a high Q-value when they are unloaded and the Q-value must not become too diminished by the presence of the patient. On the other hand, the coil must couple well with the region of a patient's anatomy which is to be imaged.
Signal-to-noise ratio is the ratio between those components in the electrical impulses appearing at the antenna terminals representing the detected MR signals, to the components representing spurious electromagnetic signals in the environment, and internally generated thermal noise from the patient. To optimize the signal-to-noise ratio, the antenna should have low sensitivity to signals from outside the region to be imaged. To enhance both signal-to-noise ratio and sensitivity, the antenna is "tuned" or arranged to resonate electrically at the frequency of the MR signals to be received, typically several megahertz or more. Neither the coil size nor geometry of the antenna can therefore create an inductance or self-capacitance which prevents tuning to the desired frequency.
The antenna must also meet certain physical requirements. The antenna should have a high filling factor, which maximizes the amount of tissue which fits within the volume detected by the windings of the coil. The antenna must also fit within the relatively small imaging volumes typically provided for receiving the subject within the magnet assembly, along with other components of the system and the subject. The antenna should not cause significant discomfort to the subject. Additionally, the antenna should be easy to position with respect to the subject, and be relatively insensitive to minor faults in positioning relative to the subject.
These numerous considerations often conflict with one another and together are a challenge to the antenna designer.
The sensitivity and signal-to-noise ratio of radio frequency antennas for use in MRI have been improved by providing a secondary coil, tuned to resonate at the Larmor frequency of the element of interest, for being positioned proximate the part of the subject which is to be imaged, and a similarly tuned primary coil, typically a single loop, for being positioned adjacent the secondary winding. The primary and secondary coils are inductively coupled to each other. The primary winding is connected to the pre-amplifier of the MRI system. MR signals emitted by the patient induce voltages in the secondary winding, causing current to flow within the winding. The current generates a magnetic field which induces voltage in the primary winding. The MR signals may be received by the primary coil, as well. The secondary and primary coils amplify the MR signals, and the primary coil filters spurious signals outside of the frequency band of the Larmor frequency. See, for example, U.S. Pat. No. 5,583,438 and U.S. Pat. No. 5,575,287, assigned to the assignee of the present invention.
One promising area of MRI is the diagnosis of breast cancer. MRI enables very accurate discrimination between healthy and cancerous tissue without the discomfort of more traditional techniques. In addition, MRI can be used to detect leakage of silicone from breast implants. While expensive, recent innovations can make MRI examination of the breast region of a patient economical. U.S. Pat. No. 5,490,513, for example, assigned to the assignee of the present invention, teaches a method and apparatus for increasing patient throughput, and hence the efficiency of the use of the MRI system, by providing multiple patient handling systems in an MRI system with multiple entrances to the imaging volume. In accordance with a method disclosed, while MRI is being conducted on a first patient inserted into the imaging volume on a first patient handling system through a first entrance, a second patient is being positioned and prepared for imaging on the second patient handling system. As soon as the imaging procedure is completed on the first patient, the second patient is inserted into the imaging volume through a second entrance and imaged, while a third patient is being prepared on the first or another patient handling system. A greater number of patients can thereby be imaged in a limited period of time.
Tumors can be located both in the breast tissue and in the chest wall surrounding and underlying the breast. The ability to detect tumors in the chest wall in addition to the breast tissue, depends in part on the ability of the radio frequency antenna used in the imaging process to accurately detect MR signals from the chest wall. It has also been found that MRI of the breasts is best performed with the patient in a prone position and the breasts hanging without restraint. The antenna therefore needs to be close enough to the breast tissue to adequately couple to the tissue, without compressing the breast tissue. A radio frequency antenna which uniformly detects MR signals from the breast tissue and chest wall, with low signal-to-noise ratio, without compressing the breasts would be advantageous.